Methodology development for energy auditing

Motivation

Since 2012 and the release of the EU Energy Efficiency Directive, industries must regularly assess their energy performance, either through high quality energy audits completed every four years or the implementation of an approved energy management system. In both cases, the energy consumption should be understood, the energy performance evaluated and energy savings opportunities generated and quantified. The output of such energy reviews depends on the chosen scope and level of detail, in turn a function of the dedicated time, resources and skills available for the analysis. Often, due to constraints on plant resources (e.g. finances, time, data availability) these energy reviews are conducted without an appropriate level of detail, usually resulting in “surface” analysis or optimisation which might not be needed if the system would be more carefully studied and integrated. Based on these observations, a systematic methodology to carry out energy reviews on chemical sites of all sizes was developed, centered on pinch analysis. The methodology (depicted graphically in Figure 1) makes use of state-of the art techniques combined with heuristics, resulting from industrial experience generated through numerous case studies. Although focused on thermal power consumption, electricity consumption is also included and investigated, considering its high interconnectivity with thermal energy vectors (e.g cogeneration, turbomachinery, combined water and energy optimisation).

Method and results

The energy requirements are defined following a top-down approach, starting from raw utilities consumption (“black box”) and gradually translated into real process heating and cooling requirements (“grey box”). To do so, the system is broken down into smaller subparts corresponding to the energy conversion units, distribution networks (e.g steam network) and production units, themselves being represented by block flow diagrams. Specific mass and energy balances around each system are carried out to validate measurements, using data reconciliation techniques when possible.

Process heating and cooling requirements are listed using the dual representation. For each process stream, the corresponding utility stream is defined, ensuring the closure of mass and energy balances and the simultaneous generation of process and utility composite curves when carrying out heat integration. The data acquisition and treatment phases to generate the list of streams are often difficult and time-consuming steps; therefore, heuristic criteria are defined to limit data size and complexity without losing information.

The generation of the system composite curves allows quantification of the improvement potential and to identify energy saving opportunities linked to better process and utility integration starting from the deepest level of detail (e.g pressure modification, direct/indirect process integration, heat pumping, steam network optimisation). Energy savings obtained from these types of measures amount to between 10% and 40% of the actual energy bill, with a recurring decrease in CO2 emissions savings around 20%. Results for six production facilities considering different projects are shown in Figure 2 with the associated CO2 reduction potentials.

 

 

 

Figure 2: Savings projects generated on 6 sites, compared to theoretical optimum targets (in CO2 savings)